1. Field of the Invention
The present invention relates to frequency and time sources, and more particularly to high performance GPS-based frequency and time sources.
2. State of the Art
The NAVSTAR Global Positioning System (GPS) is a very precise satellite-based navigation system being installed by the U.S. Department of Defense. When fully operational, there will be 24 satellites orbiting the earth in 12-hour orbits. The present system provides full 24-hour service for time broadcast and two-dimensional navigation. The GPS system is the first navigation system that offers global coverage, precise navigation in all weather conditions, and spread spectrum transmission to minimize interference and terrain effects. The GPS system provides position information in three dimensions (latitude, longitude, and altitude) and provides a precise time signal.
In operation, each of the satellites is continually broadcasting its own position and its own very precise time. The object of a GPS receiver is to determine its own latitude, longitude, and altitude, and to determine time. To determine these four unknowns, the receiver must listen to and use the signals from four satellites.
The advent of commercial access to the NAVSTAR Global Positioning System (GPS) has made possible GPS-based frequency and time sources providing precision performance, stable frequency and accurate time outputs. Applications of GPS-based frequency and time sources include network communications, production test and calibration and electric power distribution. Specifically, as communications systems strive for faster and faster data rates, the concept of Bit Error Rate (BER) plays a more important role. To achieve low BERs, network synchronization is critical; that is, the frequencies at each node in the system have to be maintained more closely, more accurately, and with less drift than previously allowed.
A GPS-based time source, or GPS clock, addresses the foregoing key concerns of communications providers. Frequency, accuracy and stability are similar to those obtainable from atomic frequency standards. The GPS system offers time stability to within 300 ns. Typical pulse-to-pulse jitter in GPS timing receivers however, is 40 to 60 ns. In the design of GPS receivers, emphasis has conventionally been placed on producing a time output (1 pulse per second, or 1 pps) having a short-term average that is very accurate in relation to GPS time. Pulse-to-pulse jitter has typically not been an overriding concern. A frequency output (for example, 10 Mhz) is generated based on the time output. Such an arrangement is illustrated in FIG. 1, in which an oscillator 11 is incorporated in a closed loop frequency multiplier 13 that receives as an input a 1 pulse per second output of the GPS receiver 15 and produces therefrom a 10 Mhz frequency output signal. The GPS receiver 15 uses its own free-running local oscillator 17 to receive transmissions from GPS satellites so as to produce the 1 pulse per second time output.
The manner in which the 1 pulse per second output is produced is shown in greater detail in FIG. 2. The signal derived from the local oscillator 17 is frequency divided 21 to produce the 1 pulse per second output. Because the local oscillator 17 is free-running, its output signal will exhibit some time bias .DELTA.T (or drift) with respect to GPS time. A GPS processor 19 calculates the time bias and changes the count in a counter 21 of the frequency divider so as to correct for the time bias. This technique is known as "jamming the clock". With the correction, the short-term average of the IPPS time output will be accurate in relation to GPS time. The time output will contain considerable pulse-to-pulse jitter, however, due to the fact that it is continually "jam-set" with the consequence that the accuracy of the 10 Mhz frequency multiplier output 13 is impaired. Such jitter prevents the full benefit of a GPS-based time source from being realized.
In the field of production test and calibration, clearly, the more accurate the frequency standard, the faster calibration of a unit under test may be performed. Ideally, the frequency standard employed would be the Cesium Standard. The cost of a Cesium Standard, however, is often times prohibitive. The Cesium Standard is a primary standard that does not need to be calibrated, although its performance does need to be monitored to assure that the standard is operating properly.
Secondary standards, such as Rubidium and Quartz cost less than a Cesium Standard, although a Rubidium Standard is still quite expensive. Whether a Rubidium Standard or Quartz Oscillator, or even a Cesium Standard, is used, traceability to a National Standard reference must be maintained. A GPS-based clock is an acceptable means world-wide of obtaining the traceability needed in the calibration of frequency based devices. Furthermore, GPS-based clocks are continuously calibrated. In production test operations, therefore, the frequency source does not have to be removed for calibration.
In these and other applications, a need exists for a GPS-based frequency/time source that exhibits the greatest possible accuracy at the lowest possible cost. In communications network applications in particular, an affordable GPS frequency/time source would allow such a source to be installed at every node in a network to achieve faster data rates using the highest frequency stability standards currently achievable.